Integrated circuits (ICs) can include thousands or millions of semiconductor devices, mostly transistors. The motion of free carriers in a semiconductor leads to current. As an electric field is applied to a semiconductor, the electrostatic force causes the carriers to first accelerate and then reach a constant average velocity due to collisions with impurities and lattice vibrations. The ratio of the velocity to the applied field is called the mobility. Increasing carrier mobility in a semiconductor can have many beneficial effects. For example, increasing carrier mobility in the channel region of a Metal-Oxide Semiconductor Field Effect Transistor (MOSFET), or MOS transistor, increases the switching speed of the MOSFET.
Mechanically stressing the semiconductor can increase carrier mobility. For example, in an N-channel MOSFET (or NMOS transistor), the major carriers are electrons. Introducing tensile stress to the channel region of an NMOS transistor increases electron mobility, thereby increasing the NMOS transistor's performance. The major carriers in a P-Channel MOSFET (or PMOS transistor), on the other hand, are holes (or the vacant positions left behind by electrons freed of their covalent bonds.) Introducing compressive stress to the channel region of a PMOS transistor increases hole mobility, thereby increasing the PMOS transistor's performance.